SCIENCE. 



[N. S. Vol. XVII. No. 420. 



Now as the axis of the planet is nearly 

 perpendicular to the line of sight, we shall 

 see objects in their true dimensions only- 

 near the middle of the disk and on the 

 equator. In the revolution of the planet 

 in its orbit, the equator, as seen from the 

 earth, may be displaced 3.3 degrees. There- 

 fore, all objects seen on the disk may ap- 

 parently be shifted in latitude. At the 

 equator the displacement may amount to 

 1.1" of arc, or about one sixteenth of the 

 polar diameter, while in higher latitudes 

 it will be very much less, and at the lati- 

 tude of 70 degrees the displacement will 

 be only 0.28" of are. 



During the past twenty-five years some 

 astronomers, who have observed Jupiter 

 for years, imagine that when the planet 

 is turned with its axis three degrees toward 

 the earth, one would be able to see to the 

 pole and beyond. I may say that this is 

 a mistake, for the reason that the displace- 

 ment of three degrees would amount to 

 only 0.03" near the pole. It is very rare 

 that any objects are seen beyond 40 de- 

 grees of Jovian latitude. The latitude of 

 70 degrees is only 1" from the limb, and 

 80 degrees only 0.25" from the limb of the 

 planet. Hence objects, if they existed at 

 high latitudes, would be practically in- 

 visible. During twenty-three years of ob- 

 servation I have never observed a separate 

 marking beyond 42 degrees of Jovicentric 

 latitude, or 5.7" of arc from the limb, ex- 

 cept on one night when a small white spot 

 was seen in latitude 62 degrees, or with- 

 in 2" of the south limb of the planet. 

 Usually a fine shading or discoloration of 

 the disk is seen near the poles. The planet 

 rotates on its axis in a little less than ten 

 hours, and hence the shape and size of an 

 object in passing across the disk will be 

 materially modified by the effect of rota- 

 tion. An object, when it is first brought 

 into view on the disk by rotation, is in- 

 finitely short in length and, as it is brought 

 farther on by rotation, the length is in- 



creased, and reaches its maximum when on 

 the central meridian of the disk. In pass- 

 ing off, it of course goes through the same 

 changes in apparent size. As the meri- 

 dians on a globe are curved lines, objects 

 in passing across the disk may apparently 

 be displaced in longitude in regard to each 

 other, due to the curvature of the meridian, 

 viz., two spots lying in different latitudes 

 might at one time be on a line parallel to 

 the polar axis of the planet and, when 

 brought on the middle of the disk, would 

 lie in different longitudes. Some astron- 

 omers have been misled by phenomena of 

 this kind, considering it to be a real motion 

 of the object, when in fact it is simply 

 displacement due to rotation. 



In order then to know what phenomena 

 are real and what are apparent, it is neces- 

 sary to take into account the position of 

 the earth with regard to Jupiter's equator, 

 as well as the position of the object on 

 the disk of the planet. 



Jupiter is distant 5.2 times the distance 

 of the earth from the sun, and at mean 

 distance 1" of arc amounts to 2,300 miles. 

 Now owing to the great distance of the 

 planet from the earth, the objects we see 

 must have considerable size in order to be 

 visible. I presume that the smallest ob- 

 ject which has been observed for longitude 

 or latitude is at least 2,000 miles in di- 

 ameter. In the ease of a line or streak 

 one might be able to see with the aid of 

 the modern telescope 0.1" of arc in width, 

 which on Jupiter would be 230 miles, but 

 all the markings which have been observed 

 are considerably greater in size than this 

 minimum value. The ordinary spots we 

 see on Jupiter, from which rotation time 

 has been determined, have usually been 

 upward of 3,000 miles in diameter, where 

 the spot is circular or elliptical. 



I began systematic observations on Ju- 

 piter in the year 1879, and these have 

 been continued every year with the excep- 

 tion of the opposition of 1888 and a part of 



